Supramolecular Water Clusters Associated with Polyoxometalates

FIGURE 38. Three-dimensional network of [(DB18C6)Na(H20)i 5]2[Mo60i9] CH3CN with ID sandglass-like channels along the c axis. The acetonitrile molecules are omitted for clarity. (Reprinted with permission from ref 72.) 

SUPRAMOLECULAR WATER CLUSTERS ASSOCIATED WITH POLYOXOMETALATES 

Water is a major constituent of planet earth and it plays an important role in many natural systems. Numerous biological systems depend on diverse noncovalent interactions in which water plays major role by stabilizing the native conformation of the respective biomolecules. However, water is not a fully understood liquid due to its anomalous behavior. Thus numerous recent reports have appeared on extensive investigations of water structures. Studies on small water clusters and their noncovalent interactions are also significant because such clusters stabilize supramolecular systems both in solution and in the solid state. Therefore, there is a clear need to understand how such aggregates influence the overall structures of their surroundings.  

In many chemical systems, small water clusters are responsible for the aggregation or self-assembly of organic and inorganic molecules. The study 

FIGURE 39. The molecular structure of [ Na(dibenzo-18-crown-6)(MeCN) 3 PM012O40 ] in which the Keggin unit supports three supramolecular sodium-crown ether complexes via three terminal oxygen atoms of a single M03O13 unit of the Keggin anion. (Reprinted with permission from ref. 73.) 

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IV. SUPRAMOLECULAR WATER CLUSTERS ASSOCIATED WITH POLYOXOMETALATES... 

Detailed discussions on supramolecular structures of giant polyoxometalate clusters have been reported by Muller and co-workers." " The number of ionic lattices formed from anionic POM clusters and organic cations-coordination complex cations resulting in supramolecular structures is substantial and literally more than hundreds of new compounds in this class appear annuallyIn this overview, the supramolecular features of POM-based systems will be discussed mainly under three headings (a) supramolecular features of polyoxometalate supported transition metal complexes, (b) polyoxometalate-crown ether complexes with supramolecular cations, and (c) supramolecular water clusters associated with polyoxometalates. 

Supramolecular Water Clusters Associated with Polyoxometalates 103... 

In this volume we have collected 10 review chapters from distinguished scientists who have contributed extensively to the study and development of supramolecular assemblies that contain metals and metal-like elements with unusual structures and morphologies and possess potentially useful (and applicable) physical and biological properties. The first chapter by K. Ariga et al. is a general discussion of supramolecular structures that contain inorganic building blocks for hybrid lipid thin films, layer-by-layer assemblies, structure transcription, and functional mesoporous hybrids. This is followed by two chapters, the first by M. L. Kistler et al., who describe the self-assembly of hydrophilic polyoxometalate (POM) macro-anions and examine the structure and behavior of POM macro-ions in solution. This is followed by a chapter by S. K. Das, who provides an overview of the supramolecular features of POM-supported transition metal complexes, POM-crown ether complexes with supramolecular cations, and supramolecular water clusters associated with POMs. 

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